Fluorescent lamps are well-known in the art and are used for a variety of types of lighting installations. Such lamps are characterized as low pressure discharge lamps and include an elongated envelope whose interior surface is coated with a phosphor such as calcium halophosphate, zinc silicate or calcium tungstate, and an electrode at each end of the envelope. The envelope also contains a quantity of an ionizable medium such as mercury, and a starting gas at a low pressure, generally in the range of 1 to 5 mm Hg. The starting gas may consist of argon, neon, helium, krypton or a combination thereof.
One of the most commonly used methods for introducing mercury into such lamps is a mechanical dispensing unit which forms part of so-called exhaust machine. Mercury is dispensed by the action of a slotted plunger passing through a reservoir of mercury and into the closed exhaust chamber housing the exhaust tube. The mercury falls through the exhaust tube into the lamp. This method of dispensing mercury has many drawbacks. In the first place, the mercury dispensing unit complicates the exhaust machine. In the second place, the mercury is introduced into the lamp envelope which is at a high temperature and which is in open communication with the exhaust machine. As a result, it is inevitable that a portion of the introduced mercury evaporates and disappears from the lamp, or a portion of the filling gas is driven out of the lamp. Furthermore, the introduction of mercury through the exhaust tube involves the risk of mercury getting stuck in the exhaust tube so that after sealing off the lamp it contains too little or no mercury at all. For these reasons a large excess of mercury, namely a multiple of the quantity required by the lamp is generally introduced. Finally, working with mercury on the exhaust machine requires additional safety precautions on medical grounds.
An alternative method of dispensing mercury, as shown for example in U.S. Pat. Nos. 3,657,589 and 3,728,004, is to place inside the lamp a mercury compound that is inert under lamp processing conditions but can later be activated to release mercury. Disadvantageously, this method releases impurities, which then require special gettering. It also requires a relatively long period of time (20 to 30 seconds) to activate the mercury compound. As a result, this method of dispensing mercury does not readily lend itself to high speed production machinery.
The drawbacks described hereinbefore may be obviated by placing the mercury to be introduced into the lamp in a closed capsule mounted within the lamp, whereafter the lamp is provided with the desired fill gas and is subsequently sealed. The mercury containing capsule is not opened until all manufacturing steps relating to the exhaust process have been completed.
The above-mentioned mercury capsules are generally fabricated from glass or metal. Examples of mercury containing glass capsules are shown for example in U.S. Pat. Nos. 2,415,895; 2,991,387; 3,764,842; 3,794,402; 4,182,971 and 4,335,326. These examples require special heaters proximate the capsule or an additional capsule shield to prevent loose capsule particles within the lamp upon capsule rupture.
Examples of mercury-containing metal capsules are shown for example in U.S. Pat. Nos. 2,288,253; 2,322,421; 3,300,037; 3,895,709; 3,913,999; 3,983,439; 4,056,750 and 4,282,455; and Great Britain patent application No. 2,040,554A. The metal mercury capsules contain at least one end portion sealed by flat crimping or cold welding followed in some instances by resistance welding. The disadvantages of the above methods of sealing the end portion of the metal capsules include the inability to accurately control the desired temperature at which the mercury is released from the capsule. The type of seals used in the mercury capsule must be capable of withstanding the temperatures of the glass sealing operation during the lamp manufacturing process which can exceed 300.degree. C. Flat crimping or cold welding the mercury capsule has been found to be insufficient to repeatably contain all of the mercury within the capsule during these relatively high temperatures. Some of the methods of sealing the mercury capsule, for example, U.S. Pat. Nos. 3,895,709 and 3,913,999 which use resistance and/or cold welds, necessitate the need for expensive laser equipment to release the mercury within the capsule.
It has been discovered that in some crimping techniques, once the crimp seal is applied to the mercury-containing capsule, the seal has a tendency to open or spring back leaving a channel of approximately 0.001 inch (0.025 millimeter) due to the crimp configuration and the physical properties of the capsule material. This channel in the capsule seal prevents the retention of 100 percent of the mercury in the capsule during lamp manufacturing.